The reason ancient tunnels are a good MF spot is that it's a level 85 area, same as river of flame, chaos sanctuary and all 3 levels of worldstone keep. Nearly everything in the game can drop in a level 85 area, but the monsters are significantly easier to kill. The only things that can't drop, as far as I know, are amulets with +2 to a class's skills, and grand charms with 41-45 life. Only Diablo, Baal and Nihlathak can drop those, although you can gamble / craft +2 ammies beginning at lv 87 (level 93 guarantees high enough iLvl), and you can reroll grand charms which those bosses drop with p.gems until you get a 41+ life skiller. I've done it hundreds of times now in SP and still no luck though, lol.

My guess is Blizzard did this so that actual players would have something to do by themselves in all the games with bots hogging chaos / baal.

Anyway, it's not that more items drop, that depends on your magic find %, and on pure luck. It's that high level, desirable items like CoA, griffs, etc. can drop. High runes too. The other main lv 85 area early in the game is the pit. There are others, but they have fewer boss packs, less monsters and less monster density. Here's a list: https://diablo2.diablowiki.net/Guide:Level_85_Magic_Find_Areas_v1.10